FIELD OF THE INVENTION
The invention relates to a current amplifier and particularly to a current-mode sense amplifier for sensing the bitline of a memory.
In semiconductor memories the information stored in a memory cell has to be sensed and amplified for reading. A number of memory cells is connected to a bitline whereby a particular cell is selected for reading through a wordline. A sense amplifier connected to the bitline detects and amplifies the information stored in the selected memory cell.
A current-mode sense amplifier for a CMOS memory cell is described in an article entitled "A High-Speed Clamped Bit-Line Current-Mode Sense Amplifier", by Travis N. Blalock et al. in IEEE Journal of Solid State Circuits, Vol. 26, No. 4, April 1991, pages 13 to 19. The known current-mode sense amplifier compares an input current from a memory cell with a reference current. The reference current is provided by a dummy memory cell producing a current that is between the currents representing digital logic states. Usually, the reference level is set at a mid-point between no data current and minimum data current. The current-mode sense amplifier detects a data input current above or below the reference current as the different digital logic states.
The known sense amplifier demands a high degree of layout symmetry between the sense amplifier transistor pairs, especially with respect to element orientation or dimensioning. The transistor count is rather high and needs much chip area. Its operation is very susceptible to cross talk effects. The structural layout of the amplifier itself and of the memory cell array must consider the crosstalk risk. The sense amplifier layout must be compact to avoid significant contribution on internal node capacitances by transistor interconnections.
The sense amplifier is highly sensitive leading to the disadvantage of being susceptible to noise and process variations due to the sensitive nodes of the amplifier and its high gain feedback loop. That may affect the current margin of the sense amplifier.